This invention relates to a universal transmitter which can operate a garage door operator, a gate operator and other movable barrier operators, and more particularly to a universal transmitter which can select among a plurality of different channels and, using a single antenna loop and a single radio frequency (RF) circuit, transmit on the selected channel.
Most manufacturer-supplied transmitters designed for garage door or gate applications are single function, single frequency devices with a preset carrier frequency and use either a switch-selectable code or a preset factory code. Switch-selectable codes are set by the user setting a plurality of switches on the transmitter and the receiver units. Factory-set codes are input into the receiver by causing a microcontroller or other processor such as a microprocessor, gate array or the like, within the receiver to perform a learn function. The receiver enters the learn mode, then the user activates the transmitter, which transmits a signal representing the factory programmed code stored in it. The most recent transmitters employ rolling code or other code encryption.
Each manufacturer has developed its own separate modulation format and selected its own carrier frequency. Recently, some O.E.M.""s and aftermarket manufacturers have developed transmitters which permit the generation of multiple formats and frequencies within a single transmitter.
The aftermarket for garage door, gate and other movable barrier operator remote transmitters is brisk. As manufacturers improve their products by offering greater functionality, the cost of providing replacement parts for older model units increases. Generally, receivers have a longer working life than remote transmitters. A goal among aftermarket providers is to furnish a single, universal transmitter which can be programmed to be used in a multitude of systems from different manufacturers.
The difficulty of designing a universal transmitter which can operate at multiple frequencies for multiple code types, while keeping manufacturing costs down is the aftermarket supplier""s greatest challenge.
U.S. Pat. No. 5,564,101 to Eisfeld et al. discloses a system having a plurality of complete transmitter circuits for generating a plurality of difference RF carrier frequencies to operate a plurality of different receivers. The transmitter includes two sets of mechanical switches or DIP switches by which the user sets the transmitter code and the carrier frequency. A separate oscillator and an antenna is provided for each user-selected RF carrier frequency.
U.S. Pat. No. 5,661,804 to Dykema et al. discloses a learning transmitter which can operate a plurality of different receivers which employ a rolling or encrypted code. No user input is required to learn the code and frequency, other than activating the transmitter to be copied. A single RF circuit and dynamically tunable antenna is provided for transmitting the learned code. The single RF circuit employs a phase locked loop frequency synthesizer and separate control logic for outputting the learned frequency and code.
While both of these system are capable of operating a plurality of receivers, each is complex and expensive. There is a need for an inexpensive, simple, universal transmitter capable of operating a multitude of different receivers at different frequencies. There is a need for a universal transmitter which uses a single transmitter circuit, using simple components, for transmitting a plurality of different carrier frequencies.
A radio frequency transmitter according to the invention provides a unique combination of inexpensive and simple circuits. It is compatible with a large number of garage door, gate and barrier operators manufactured by different manufacturers. The radio frequency transmitter can be programmed to activate a plurality of movable barrier operator receivers, each receiver receiving a particular carrier frequency modulated according to a particular modulation scheme.
The RF transmitter includes a single transmitter circuit for transmitting a signal at a plurality of different carrier frequencies according to a plurality of different modulation codes. The single transmitter circuit includes a transmit oscillator, a tuning circuit comprising a plurality of discrete reactive components, a radiating element having a variable length, and a control circuit coupled to the tuning circuit, the transmit oscillator and the variable radiating element. The user inputs a desired carrier frequency and a desired modulation code through a plurality of switches. These values are stored in a programmable controller. A particular carrier frequency and code can be assigned to each switch. In a preferred embodiment, the transmitter includes three user switches for operating up to three different barrier operators.
A programmable controller is coupled to the transmitter circuit for operating the transmitter circuit to cause the transmitter circuit to be modulated with signals generated by the programmable controller from the stored user-selected carrier frequency and the stored user-selected modulation code. Specifically, the programmable controller provides the logic to select the particular reactive elements in the tuning circuit, transmit oscillator and to vary the electrical length of the radiating element. The variable length radiating element is operable for radio frequency transmission of the signals generated by the programmable controller.
Preferably the programmable controller provides logic control to PIN diode switches for shorting in or out selected reactive elements and for varying the electrical length of the loop antenna element. Specifically, the PIN diodes are used to short out various capacitors in the tuning circuit and the transmit oscillator circuit. When not selected, preferably the PIN diodes are reverse-biased. While in the off state, the PIN diodes have a high impedance and low capacitance. This minimizes stray parasitic transmissions.
A single transmitter circuit is used for all RF frequencies. Two or more of the external switches are used for programming in the manufacturer""s carrier frequency and to set the transmitter""s code. Preferably the variable length radiating element is a loop formed as a trace on a printed circuit board.
In a single RF circuit switching to obtain multiple carrier frequencies is relatively straightforward. It may be difficult, however, to eliminate harmonics that are prohibited by FCC standards. Elimination of harmonics is achieved through positioning of the reactive elements of the transmit oscillator circuit, the tuning elements and the radiating element located on the printed circuit board. PIN diodes are used to short across capacitors instead of switching in and out of the circuit. This has the advantage of eliminating interaction between the components.
Lead lengths between the components in the transmit oscillator and tuning circuits are made as short as possible to minimize changes from board to board during manufacturing. Capacitive elements are positioned on opposite sides of the printed circuit board to cut down on parasitic harmonic radiation. Selected elements in the transmit oscillate circuit, as well as the loop antenna, form part of the radiating element.